Process for optical communication and system for same

ABSTRACT

The required penalty in the optical signal-to-noise ratio induced by nonlinear effects in an optical communication system is reduced by specific expedients. The communication system is operated and is adapted to be operated in a pseudo-linear regime. Further, an optical phase conjugator is employed with a suitable dispersion map. This combination yields a desirable improvement in the required penalty in the optical signal-to-noise ratio due to nonlinear effects.

FIELD OF THE INVENTION

[0001] This invention relates to optical communication and in particularto optical communication in which nonlinear optical effects aresignificant.

ART BACKGROUND

[0002] In optical communication systems a signal is 1) launched on anoptical line or path, 2) optically amplified periodically (discreteamplification and/or distributed Raman amplification) along the line, 3)optically reshaped and/or retimed for further transmission and 4)received and converted into a signal in electronic form. (Reshaping isdefined as any process that is optical and by which the optical signalis transformed such that after transformation the variation in amplitudeamong signal portions corresponding to a code level (e.g. 0 or 1 in thecase of binary code) is reduced and retiming is defined as any processthat reduces the variation of any individual coded level of a signal atoptimum sampling time used to detect a bit such that the overall biterror rate for bits at such level is minimized.) Thus the line isdefined by a series of segments with each segment attached at each endto a device i.e. amplifier (discrete amplifier or a pump for adistributed amplifier), transmitter, multiplexers, demultiplexers,filters, wavelength converters, dispersion compensators, receiver,retimer, switch, add-drop multiplexers, cross connects, modulator, orreshaper. For example, in FIG. 1 segment 7 has a reshaper anderbium-doped fiber amplifier (EDFA) attached to its end points; segment8 has an EDFA at its end points; segment 9 an EDFA and Raman amplifierpump; while segment 10 has a Raman pump and transmitter at its endpoints.

[0003] The signal injected into the line is substantially affected byits propagation along the line. Both linear and nonlinear phenomenacontribute to this change in the signal. A significant contributor tosuch linear effects is chromatic dispersion. That is, differentwavelengths of light travel along the line at different speeds. Since nopulse of light is perfectly monochromatic, all pulses will be broadenedas they traverse the line because the longer wavelength components willtravel along the line at a different speed from the shorter wavelengthcomponents. Accordingly, an injected narrow pulse will be received atthe end of a segment as a broadened pulse with longer wavelengths on oneside of the broadened pulse and shorter wavelengths on the other. Theproperties of the segment determine whether the longer wavelengths, orshorter wavelengths, travel faster.

[0004] Nonlinear effects also influence injected pulses. For nonlineareffects the intensity of the signal affects the speed at which differentparts of the signal propagates and/or causes interactions (e.g.exchanges of intensity) between portions e.g. pulses, of the injectedsignal. In single frequency channel communication systems suchinteractions are significant and occur between pulses while inmultichannel communication systems where a plurality of signals areinjected each within a different wavelength range, such interactionsboth between frequency channels and within a signal frequency channelare quite significant.

[0005] The result of these linear and nonlinear effects is that onepulse upon propagation overlaps with another by spreading of the pulseover the other and/or pulse distortion is induced by transfer of energyfrom one pulse to another. Accordingly either a slower bit rate (greaterpulse spacing) or a shorter transmission distance between reshapingand/or retiming must be used to prevent loss of information. Slowing ofthe bit rate or more frequent amplification is not desirable due to theassociated increase of capital costs of the system.

[0006] A variety of expedients have been developed to reduce theconsequences of such linear and nonlinear phenomena. For example,dispersion compensators have been inserted into optical systems tomitigate linear effects. After chromatic dispersion is introduced bypropagation of an optical signal through all or part of a line, thesignal is transmitted through a fiber coil that introduces dispersionopposite in sign to that accumulated in the line. For example, a segmentor series of segments of the line speeds up (or slows down) the shorterwavelengths relative to the longer while the coil slows down (or speedsup) the shorter relative to the longer wavelengths. As a result, thechromatic dispersion of the line is offset and thus, in essence,removed.

[0007] Other innovations have reduced signal degradation associated withnonlinear effects. In one particularly important approach the opticalfiber (denominated TrueWave® by OFS Fitel, Inc.) of the line isconfigured to have a non-negligible dispersion e.g. 5 psec/(nm·km)rather than manufactured to minimize such dispersion. While theintroduced dispersion is corrected after propagation as previouslydiscussed, it helps reduce nonlinear effects. By purposefullyintroducing chromatic dispersion signal pulses in different channels(and thus different wavelengths) are caused to traverse the line atdifferent speeds. The pulses from different channels, thus, acquiredifferent phases during propagation resulting in phase mismatch amongchannels that mitigates four-wave mixing (a nonlinear effect).Accordingly the nonlinear interaction between these pulses is limited.

[0008] Despite the improvement due to TrueWave® fiber, the high bitrates (greater than 2.5 gigabits per channel) now being employed or nowcontemplated has increased the possibility of encountering increasedpower (more pulses per unit time). Additionally the desire to increasespan distances between amplifiers has made use of increased powerappealing. Increased power however, as previously discussed, leads tomore pronounced problems involving nonlinear effects.

[0009] Further inventive approaches have been developed to address thesedifficulties at high bit rates. As discussed in Kaminow, I., et.al.(2000) Optical Fiber Telecommunications IV B, Chapter 6, Academic Press,New York, pages 232-304, ISBN 80-12-395173-9; dispersion mapping is onesuch approach. In one embodiment rather than launching narrow pulses,optical signals are processed so that they have, when injected, the samepulse broadening as a narrow pulse would have if it had undergonesubstantial linear dispersion. The configuration of the signalcorresponding to a particular level of dispersion applied to a narrowpulse is adjusted periodically throughout the line. For example, FIG. 2is illustrative of a dispersion map. The map is a graph of positionalong the line versus the waveform of the signal at this position asrepresented by the degree of cumulative dispersion produced by lineareffects required to produce such waveform when applied to a signal withnarrow pulses. Thus, the map at 6 in FIG. 2 shows a cumulativedispersion of −800 psec/nm upon injection.

[0010] In the example of FIG. 2 the injected signal having −800 psec/nmcumulative dispersion, is very broad, rapidly changes configurationuntil it has sharp pulses at zero cumulative dispersion (point 21 inFIG. 2), and further rapidly changes configuration until it again isquite broad at +800 psec/nm (point 22 in FIG. 2). Because of the rapidchanges in configuration the induced phase change is averaged over theentire pulse in a uniform manner, and thus significant distortion ofeach pulse is avoided. The linear dispersion is adjusted usingdispersion compensators typically at each point of amplification(indicated in FIG. 2 by 17) to produce the desired dispersion to reducenonlinear effects. Because of this averaging and intensity mitigation,nonlinear effects as previously discussed, are reduced. The particulardispersion map employed depends on the specific properties of the line,the bit rate, and the power of the injected pulse. (See Kaminow supra,Chapter 6 for a discussion of the considerations involved in choosing adispersion map.)

[0011] An alternate expedient has been employed to continue the drivetowards reducing the penalty in required optical signal-to-noise ratiointroduced due to nonlinear effects. (Referred to for purposes of thisinvention as required OSNR penalty.) In this regard a particular systemincluding an optical phase conjugator (OPC) has been described. (SeeBrener, I. et.al. “Cancellabon of all Kerr Nonlinearites in Long FiberSpans Using a LiNbO₃ Phase Conjugator and Raman Amplificabon,” OpticalFiber Communication Conference 2000 postdeadline paper PD 33-1.) An OPCalso has the property of reversing the sign of the cumulative dispersionof a signal associated with linear effects (e.g. +800 psec/nm becomes−800 psec/(nm) and exchanging the longer wavelengths of the pulse withthe shorter wavelengths. Thus, as shown in FIG. 3, the shorterwavelengths, 41, and the longer wavelengths, 42, of a pulse areinterchanged as shown in FIG. 4. The field generated by distortingeffects leading to distortion is phase conjugated (part of the phaselinear in distance along the line undergoes a sign change) by the OPC.Upon propagation over a second segment of a line because of theconjugation, such distortion is gradually but increasingly mitigated.The power distribution has an effect on the extent of this mitigation.(The power distribution is the graph of the total power of the signal,versus position along the line.) An asymmetric power profile with an OPChas been considered and viewed as disadvantageous especially compared toa symmetric or nearly symmetric power distribution (as produced byemploying distributed Raman amplification). As previously discussed, theamplitude of nonlinear effects is dependent on signal intensity. In thecase that the intensity distribution along the line is symmetric ornearly symmetric around the OPC, the penalty associated with nonlineareffects generated in the segment following the OPC offsets the nonlineareffects produced in the segment before the OPC.

[0012] The use of OPCs has also been employed in other manners to reducedistortion. Specifically, in a soliton transmission system, it ispossible to position an OPC so that timing jitter due to noise issignificantly reduced. (See Smith, N.J. (1997) “Soliton TransmissionUsing Periodic Dispersion Compensation,” Journal of LightwaveTechnology, 15(10), 1808 for a description of such approach.)

[0013] Despite all such improvements, it is always desirable to reducefurther the required OSNR penalty introduced by a segment or series ofsegments and thus allow higher pulse rates and/or greater signalintensities.

SUMMARY OF THE INVENTION

[0014] The penalty associated with nonlinear effects is reduced whilenot substantially affecting those associated with linear effects by thepractice of the invention. Significantly, this result is accomplishedwithout ensuring that the power profile is symmetric on either side ofan OPC. It is possible for the power profile to be symmetric orasymmetric around the position of the OPC. In particular at least oneOPC is employed somewhere within a series of segments. However, the OPCshould be used in conjunction with a particular class of dispersion mapsin a pseudo-linear operating regime. For operation in a pseudo-linearregime three criteria are satisfied. First, the bit rate should be 20gigabits/sec or greater for at least one channel. Second, somewherewithin a series of segments being improved, the temporal full width athalf maximum (FWHM) of a pulse becomes 2/B where B is the bit rate. Forexample, 1/B is 25 psec for a bit rate of 40 gigabits per second. Third,in the series of segments to be improved the power for at least onechannel having a bit rate of at least 20 gigabits per second reaches atleast one tenth the power launched from the transmitter that is thesource of signals for that channel in the segment. The dispersion mapemployed is also significant. For the series of segments being improved,the dispersion map is configured so that the absolute value of the ratiobetween a) the sum of positive dispersions at half points and b) the sumof the negative dispersion at half points, is in the range 0.5 to 2.0preferably 0.8 to 1.25, most preferably 0.9 to 1.1. (The half point, Zofor a segment is the point Zo along the segment where ∫o^(Zo)γ(z)P(z)dzequals ∫_(Zo) ^(L)γ(z)P(z)dz. (P(z) is the function of signal powerversus position in a segment, L is the length of the segment, and γ(z)is a coefficient as defined in Kaminow supra, page 248 equation 6.25which states that γ equals n₂ω₀/(cA_(eff)) where n₂ is the Kerrnonlinear refractive index coefficient of the fiber, A_(eff) is theeffective mode area of the fiber, ω₀ is the angular frequency oftransmitted light and c is the speed of light in vacuum.) That is, asshown in FIG. 5 the area 72 under the curve, 74, P(Z) (with theassumption for purposes of this illustration that γ(z) is constant forall z), on the left of dotted line 75 is equal to the area, 73, underthe curve to the right of 75. The OPC should be positioned within thesegment or series of segments to be improved, for example at a positionbetween the half point positions corresponding to negative dispersionand half points corresponding to positive dispersion. Some dispersionmaps are configured to provide low magnitude excursions of thecumulative pulse dispersion from zero. In these maps, the cumulativedispersion at half point positions of segments is kept below apreselected maximum value. The preselected maximum values in picosecondsper nanometer are less than about 16,000 to 32,000 times the inverse ofthe bit rate in gigabits per second. At bit rates of 40 gigabits persecond, the preselected maximum values are less than about 400 to 800picoseconds per nanometer. Additionally, it is desirable for the OPC tobe positioned alone or in combination with dispersion compensators sothat the magnitude of the cumulative dispersion due to linear effects inthe presence of the OPC at the end of the line before reshaping orretiming is adjusted, for example, by using a compensator to have acumulative dispersion less than or equal to about 250 psec/nm.

[0015] To avoid substantially affecting dispersion due to linear effectsit is possible to place the OPC(s) in the line so that the dispersionmap is not altered. For example, if the desired dispersion map is shownin FIG. 6, it is possible for the waveform at 86 to encounter an OPC.The OPC reverses the sign of the cumulative dispersion without the needfor a dispersion compensation and the signal configuration is changedfrom point 86 to point 89. Similarly by placing the OPC at point 82where dispersion due to linear effects is zero, the conjugation due toan OPC leaves the dispersion map unchanged.

[0016] The invention provides the potential for increasing bit rateand/or increasing injected signal power through use of at least oneappropriately positioned OPC without the necessity for adjusting thepower distribution along the line by, for example, distributed Ramanamplification. (Nevertheless, use of Raman amplification with theinvention is not precluded.) An OPC is a well studied device (seeFisher, Robert A. (1995). Optical Phase Conjugation. San Diego, AcademicPress.) and does not substantially complicate the system construction.Additionally, the positive consequences associated with choice of asuitable dispersion map and dispersion compensators are not compromised.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is illustrative of an optical communication system and itsrelationship to the invention;

[0018]FIGS. 2 through 4 exemplify concepts associated with opticalcommunication and their use in the invention;

[0019]FIGS. 7, 8 and 9 relate to the concept of half point anddispersion map ratio;

[0020]FIG. 10 demonstrates effects associated with optical phaseconjugators; and

[0021]FIG. 11 is illustrative of systems relating to the invention

DETAILED DESCRIPTION OF THE INVENTION

[0022] As discussed the invention is applicable both to single andmultiple optical communication systems. For the purposes of thisinvention a system includes a system portion, i.e. a series of at leasttwo adjacent segments. The primary elements of a segment are opticalfiber waveguides. Other devices such as amplifiers, filters, wavelengthconverters, dispersion compensators, retimers, reshapers, multiplexers,demultiplexers, add-drop multiplexers, cross-connects, receivers,switches, modulators and transmitters define the end point of a segmentbut are not considered part of the segment. For example, such devicesare not parts of segments for purposes of determining the ratio ofdispersion at half points.

[0023] As discussed, the invention is effective for system portionsoperating in a pseudo-linear regime. For a system portion to beconsidered operating in such regime the following criteria should besatisfied: 1) the pulse bit rate should be 20 gigabits/sec or greaterfor at least one channel, 2) somewhere within the system portion theFWHM of a signal pulse is 2/B and 3) in the system portion the power ofat least one channel of the signal with bit rate greater than 20gigabits/sec reaches at least one tenth the power launched from thetransmitter that is the source of the channel for that system portion. Asystem portion is considered as a pseudo-linear regime system portion iffor such series of segments all three criteria are satisfied. In thisregard, a series of segments is that which forms a string of adjacentsegments. Thus, in FIG. 1, segments 12, 7, 8, 9, 11 form a series ofsegments as does 8, 9, and 10, 9, 8, 7. Analogously the system portionis configured to operate in a pseudo-linear regime if the segments andthe devices in the optical path connecting segments are configured sothat it is possible to satisfy the three requisite criteria. So as shownin FIG. 11(a) the system portion in one embodiment involves an ERDA, 134and 136, an OPC, 135, and a reshaper/retimer, 137 in the optical pathwith segments 131, 132 and 133 where the device 134, 135, 136, and 137are configured for the pseudo-linear regime. Similarly FIG. 11(b) showsthe system with Raman pumps for some amplification.

[0024] The dispersion map within a pseudo-linear regime over at leastone system portion should satisfy a certain criterion. In particular forsuch a region the absolute value of the ratio between 1) the sum ofpositive dispersions at half points and (2) the sum of negativedispersions at half points, is in the range 0.5 to 2.0. (The absolutevalue of this ratio for this invention is denominated the dispersion mapratio). So, for example, in FIG. 7 amplification occurs at points 97whose segments are denoted 91, 92, 93, and 94. The half points are thepoints 96. Thus the dispersion map ratio is the absolute value of theratio between the sum of the dispersion at points 103 and 104 to the sumat points 101 and 102. The procedure for determining the dispersion mapratio in a region having distributed Raman amplification would be thesame except the power graph would probably look more like that of FIG. 9where Raman pumps are located at points 111, the segments are 112, 113,114, and 115, and the half points are at 116. The evolution of thesignal power P(z) in a Raman amplifier is obtained by calculation asdescribed in accordance with published procedures. In particular thiscalculation is described in Essiambre, R.-J. et.al. “Design ofBidirectionally Pumped Fiber Amplifiers Generating Double RayleighBackscattering.” IEEE Photonics Technology Letters, 14(7), 914-916(2002). Computer programs suitable for performing such calculationsinclude VPI Systems Incorporated™ transmission suite software such asVPI Transmission Maker. (Cruz Plaza, 943. Holmdel Road, Holmdel, N.J.07733), and RSoft Corporation amplifier and transmission software(Ossining, N.Y., USA). The technology used to produce the OPC isgenerally not critical. Typically, an OPC is formed in a crystal ofperiodically poled lithium niobate as described in Fejer, M. M. et.al.IEEE Journal of Quantum Electronics, 28, 2631 (1992). The OPC generallyis pumped in the wavelength range 1500 nm to 1650 nm. Other OPC devicessuch as semiconductor optical amplifiers are described in Girardin,et.al. “Low-Noise and Very High-Efficiency Four-Wave Mixing in1.5-mm-Long Semiconductor Optical Amplifiers,” IEEE Photonics TechnologyLetters, 9(6) 746 (1997).

[0025] An OPC also inverts the channels of a multichannel system arounda frequency associated with the pump source frequency of the OPC. (Fordevices whose operation is based on a four-wave mixing mechanism orcascaded three-wave mixing the signal frequency is mirrored around thepump frequency. For devices whose operation is based on a three-wavemixing mechanism without cascading, the signal frequency is mirroredaround half the pump frequency. See Chou, M. H. et.al. “1.5-pm-BandWavelength Conversion Based on Cascaded Second-Order Nonlinearity inLiNbO₃ Waveguides,” IEEE Photonics Technology Letters, 11, 653 (1999)for a description of devices whose operation is based on a cascadedthree-wave mixing mechanism.) Therefore as shown in FIG. 10, channels121, 122, and 123 having frequencies as shown before traversing the OPCwould have corresponding frequencies 125, 126, and 127 after traversingthe OPC assuming the pump for the OPC is at frequency 124. As a result,the frequency order of the channels is reversed and the channelfrequencies are changed. If these changes are unacceptable aconfiguration that does not cause such a reversal is useful. Such aconfiguration is described in U.S. application Ser. No. ______(Chowdhury 6-9) filed concurrently by Aref Chowdhury and Rene' Essiambrewith this application whose disclosure is hereby incorporated byreference in its entirety. In such embodiment involving the invention ofthis application the pump wavelength of the OPCs employed are chosen asdescribed therein to avoid the channel reversal consequences discussed.

[0026] Generally it is convenient to locate the OPC at, for example, asegment end point so that access to the line is easily achieved. It isoften convenient to locate an optical phase conjugator at one of thesepositions on the dispersion map. It is possible to use more than one OPCsuch as at position 86 and 83 in FIG. 6 with the dispersion mapcontinuing to repeat the pattern beyond 83. Nevertheless the OPCs shouldbe configured, if necessary with other optical devices, such that thedesired dispersion map is not compromised. Again, as shown in FIG. 6, itis possible to position an OPC so that it encounters the waveform at 86,since a property of the OPC is that it reverses the sign of thecumulative dispersion. Thus an OPC will provide the change from point 86to point 89 on the dispersion map. As a result a dispersion compensatorat these points is not necessary to achieve the desired dispersion map.

[0027] It is possible to position OPCs in other configurations and stillnot disturb the desired dispersion map. For example, if the OPC isplaced at point 82 where the cumulative dispersion is zero the OPCcauses, no change of the cumulative dispersion occurs and the map isundisturbed. Similarly it is possible to position the OPC at a non-zerocumulative dispersion position and bring the reversal of the cumulativedispersion produced by the OPC back to its original value using adispersion compensator. Thus if an OPC is placed at position having −20psec/nm level of cumulative dispersion is inverted to +20 psec/nm and adispersion compensator would be needed to bring the level back to −20psec/nm.

[0028] As with other optical communication systems, the optical elementscomprising the line are advantageously chosen so that the magnitude ofcumulative dispersion, is compensated such that the signal beforereshaping and/or retiming has a value less than 250 psec/nm. Asdiscussed, various forms of amplification are employable. Both discreteamplifiers such as erbium-doped fiber amplifiers (EDFAs) and continuousamplification as achieved with distributed Raman amplification areacceptable. It is acceptable for all amplifiers in the system to bediscrete amplifiers such as EDFAs, to be distributed Raman amplifiers,or a combination of both. The power profile relative to the OPC positionis not critical to the invention.

We claim:
 1. A system for optical communication system comprising asystem portion comprising at least two segments and an optical phaseconjugator, said system portion configured to be a pseudo-linear regimesystem portion, wherein for said system portion the dispersion map ratiois in the range 0.5 to 2.0.
 2. The system of claim 1 wherein said systemfurther includes a receiver and said receiver comprises an opticalsignal to an electrical signal converter.
 3. The system of claim 1wherein said system further includes a receiver and said receivercomprises an optical signal regenerator.
 4. The system of claim 3wherein said optical signal regenerator reshapes said optical signal. 5.The system of claim 3 wherein said optical signal regenerator retmessaid optical signal.
 6. The system of claim 3 wherein said opticalsignal regenerator both retimes and reshapes said optical system.
 7. Thesystem of claim 1 wherein said system portion includes a fiber having adispersion magnitude in the range 2 to 100 picoseconds/(nm·km).
 8. Thesystem of claim 1 wherein said dispersion map ratio is in the range 0.8to 1.25.
 9. The system of claim 8 wherein said dispersion map ratio isin the range 0.9 to 1.1.
 10. The system of claim 1 wherein said opticalphase conjugator is combined with a dispersion compensator.
 11. Thesystem of claim 1 wherein said system portion includes a multiplicity ofoptical phase conjugators.
 12. The system of claim 1 wherein the powerprofile of the system portion is produced by discrete amplification. 13.The system of claim 1 wherein said system includes a multiplicity ofoptical amplifiers.
 14. The system of claim 1 including a distributedRaman amplification source.
 15. A process for operating an opticalcommunication system comprising propagating an optical signal through asystem portion comprising at least two segments and an optical phaseconjugator characterized in that 1) said system portion is operated in apseudo-linear regime and 2) the system portion has a dispersion mapratio in the range 0.5 to 2.0.
 16. The process of claim 15 wherein saidsystem includes a receiver that comprises an optical signal to anelectrical signal converter.
 17. The process of claim 15 wherein saidsystem includes a receiver that comprises an optical signal regenerator.18. The process of claim 17 wherein said optical signal regeneratorreshapes an optical signal.
 19. The process of claim 17 wherein saidoptical signal regenerator retimes an optical signal.
 20. The process ofclaim 17 wherein said optical signal regenerator retimes and reshapes anoptical signal.
 21. The process of claim 15 wherein said segmentcomprises a fiber having a dispersion in the range 2 to 100 psec/(nm·km)22. The process of claim 15 wherein said dispersion map ratio is in therange 0.8 to 1.25.
 23. The process of claim 15 wherein said dispersionmap ratio in in the range 0.9 to 1.1.
 24. The process of claim 15wherein said optical phase conjugator includes a dispersion compensator.25. The process of claim 15 wherein said system portion includes amultiplicity of optical phase conjugators.
 26. The process of claim 15wherein the power profile is produced by discrete amplification.